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Magnetic Compass Turning Errors

When flying, if the heading indicator becomes inoperative, it may become necessary to base your headings on the use of the magnetic compass. This is of increased importance if you are flying under IFR conditions where you do not have the option to use visual reference points on the ground for navigation. To determine the true magnetic heading of the aircraft each of these variations must be calculated and considered during flight. While it may seem at many times that the magnetic compass is somewhat unreliable to the novice pilot, by understanding the errors and learning how to compensate for them, a pilot can determine a very accurate magnetic heading from the magnetic compass that is inside the aircraft.

Magnetic Deviation and Variation:
Each plane, on the magnetic compass, should have a placarded correction value for known headings for the aircraft. This is a correction for the magnetic difference from what a true magnetic value at the location should be will allow you to correct for errors that the instrument shows. These errors are commonly resultant from magnetic fields internal to the aircraft or installation error and are usually very minor such as a degree or two for a heading.
Magnetic Heading for difference from Agonic Line
(Difference between true and magnetic north):
The amount of correction for this consideration is charted on both IFR and VFR charts. When east of the Agonic Line, add the amount of deviation from your true heading to find your magnetic heading. When west of the Agonic Line, subtract the amount of deviation from your true heading to find your magnetic heading. You can use the mnemonic "East is Least, West is Best" to remember when to add and subtract deviation.
Magnetic Dip:
Turning Error - The compass leads in the south half of a turn and lags in the north half of a turn. Remember UNOS: Undershoot North, Overshoot South. Another way to remember this is the acronym "Lead North, Lag South". When starting a turn from a northerly heading, the compass lags behind the turn. When starting a turn from a southerly heading, the compass leads the turn. This is applicable when traveling on north or south headings only.
ANDS - A memory jogger for the effect of acceleration error is the word "ANDS": Acceleration causes an indication toward North; Deceleration causes an indication toward South. This is most noticeable when traveling on an east or west heading.

When traveling on east or west headings, un-accelerated or -decelerated turns will be result in a relatively reliable indication on the magnetic compass.

To find a compass course when the true course is known, follow this equation:

True Course +- Variation = Magnetic Course +- Deviation = Compass Course

*Note: This applies for flight in the Northern Hemisphere, and would be opposite for flight in the Southern Hemisphere.
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